1. Field of the Invention
The present invention relates to an image forming apparatus and method for forming a color image by superimposing a plurality of toner images of different colors on top of each other on an image carrier such as a transfer belt, a transfer drum or a transfer sheet.
2. Description of the Related Art
As the image forming apparatus of this type, there is known one disclosed in Japanese Unexamined Patent Publication No. 2002-333756, for example. The apparatus forms a full color image by superimposing toner images of four colors of yellow (Y), magenta (M), cyan (C) and black (K) on an intermediate transfer belt, or forms a monochromatic image using a black (K) toner alone. The apparatus is provided with a rotary developing unit wherein developers of four colors are arranged radially. The developing unit is adapted to make changeover of the developers by selectively transferring any one of the developers to a predetermined development position where the selected developer is pressed against a latent image carrier such as a photosensitive member, or opposes the latent image carrier via a predetermined gap therebetween. The developer positioned at the development position develops a latent image on the latent image carrier so as to form a toner image, while the toner image is transferred onto the intermediate transfer belt moved in a given direction (image forming process). The apparatus is arranged such that after power-on or completion of the preceding printing operations, the rotary developing unit is rotated to be positioned at a predetermined standby position (home position) to wait for a printing command. The rotary developing unit waits for the printing command as positioned at the standby position.
When a color print command is applied from an external apparatus, for instance, toner images of four different colors are formed by performing the image forming process at each changeover from one developer to another and are superimposed on top of each other on the intermediate transfer belt so as to form a color image (color print mode). Specifically, the rotary developing unit is rotated to position the developer of black, as an initial color, at the predetermined development position. Then, the latent image on the latent image carrier is developed with a black toner so as to form a black toner image, which is primarily transferred onto the intermediate transfer belt. Subsequently, the same procedure as that for forming the black toner image is repeated in cycles thereby forming toner images of cyan (C), magenta (M) and yellow (Y) in this order. The resultant toner images are superimposed on top of each other on the intermediate transfer belt thereby forming a color image. After the completion of the development of the toner image of the final color, which is yellow, the rotary developing unit is returned to the standby position. When a monochromatic print command is applied from the external apparatus such as a host computer, on the other hand, the developer of black is transferred to the predetermined development position where the developer performs the image forming process for forming a monochromatic image on the intermediate transfer belt (monochromatic print mode).
In a case where the image forming apparatus of this arrangement forms an image of a relatively small size, a plurality of such images are arranged on the intermediate transfer belt. In conventional printers (image forming apparatuses), various types of which have been provided and which are capable of forming an image of A3 size in maximum, an arrangement is made such that two A-4 size images are formed and arranged in side-by-side relation on the intermediate transfer belt. Thus is achieved an increased throughput.